It is possible to translate on your own server and transfer, import the files, but i would advise against it. Once you are used working in the language portal, I think you will love the nice little featurs it has everywhere, to make translation work easy. There's things like search facility, history of English and your own language, easy ways for collaborating, merging versions... All things that are not possible to enjoy on your local machine.

As for taking effect immediatly: there is only 3 hours max delay between translating a string and having it working on your server after a simple language pack update click.